1. Field of the Invention
The present invention relates to a one-dimensional or two-dimensional photoelectric transducer in which a semiconductor circuit including a plurality of photosensors using, for example, pn junctions is formed in the same semiconductor substrate. More particularly, the invention relates to a photoelectric transducer having a circuit configuration effective for reducing noise, such as fixed-pattern noise or the like.
2. Description of the Related Art
Recently, photoelectric transducers in which photosensors and a peripheral circuit are formed on the same substrate have been actively developed.
For example, a linear sensor in which an operational amplifier and photosensors are formed on the same semiconductor substrate (The Journal of the Institute of Television Engineers of Japan, Vo. 47, No. 9 (1993), pp. 1180), an image sensor having a sample-and-hold circuit (Japanese Patent Laid-Open Application (Kokai) No. 4-223771 (1992), and a solid-state image pickup device having an internal reference-voltage generation circuit configured by an operational amplifier (Japanese Patent Laid-Open Application (Kokai) No. 9-65215 (1997) have been proposed.
In general, a bias current for an operational amplifier is generated using a constant-current-source circuit. When forming such a constant-current-source circuit using MOS (metal oxide semiconductor) transistors, for example, a CMOS (complementary MOS) constant-current-source circuit as shown in FIG. 1 (R. Gregorian, G.C. Temes: Analog MOS Integrated Circuits for Signal Processing, p. 127, FIG. 4.5. ) is generally used. A CMOS constant-current-source circuit as disclosed in Japanese Patent Laid-Open Application (Kokai) No. 7-44254 (1995) has also been proposed.
The CMOS constant-current-source circuit shown in FIG. 1 is obtained by performing tandem connection of an upper-stage current mirror circuit and a lower-stage current mirror circuit between a power supply VDD and the ground. A constant-voltage-source output V01 is obtained from a connection point of the upper-stage current mirror circuit and the lower-stage current mirror circuit. That is, the upper-stage current mirror circuit includes a pMOS transistor Q4, whose source is connected to the power supply VDD, and a pMOS transistor Q3. The gates of the pMOS transistor Q4 and the pMOS transistor Q3 are connected to each other, and the gate and the drain of the pMOS transistor Q3 are directly connected, so that each drain supplies the load side with the same current Ibias. The lower-stage current mirror circuit includes an nMOS transistor Q1, whose source is connected to the ground, and an nMOS transistor Q2 whose source is connected to the ground via a resistor R. The gates of the nMOS transistor Q1 and the nMOS transistor Q2 are connected to each other, and the gate and the drain of the nMOS transistor Q1 are directly connected, so that each drain obtains the same current from the load side. A constant current by the current mirror circuit passes through the resistor R. An output voltage provided by a voltage generated in the resistor R and the source-drain voltage of the nMOS transistor Q2 is output as the constant voltage V01 of this voltage source.
However, the inventors of the present invention have found that in conventional photoelectric transducers in which a CMOS constant-current-source circuit and photosensors are formed in the same semiconductor substrate, the CMOS constant-current-source circuit generates fixed-pattern noise.
When a voltage is applied to the gate of a MOS transistor, and a voltage is applied between the source and the drain in a state in which a channel is formed, carriers are accelerated by the concentration of the electric field near the end of the drain in the channel, to generate avalanche multiplication. Most of the carriers generated by the avalanche multiplication result in a substrate current. Light emission occurs in the process of recombination of such hot carriers. New electron-hole pairs are generated in the same semiconductor substrate by the emitted light. A part of the generated carriers diffuses through the semiconductor substrate and is mixed in the photosensors. Since the degree of mixture is not constant for all of the photosensors, fixed-pattern noise is generated.
FIG. 2 is a schematic diagram illustrating the manner of generation of fixed-pattern noise in a one-dimensional photoelectric transducer having a conventional CMOS constant-current source, together with a plan view of the semiconductor substrate. In FIG. 2, a constant-current-source circuit having the same configuration as the current mirror circuits shown in FIG. 1 is provided. The upper-stage current mirror circuit includes pMOS transistors, and the lower-stage current mirror circuit includes nMOS transistors. These circuits are disposed in the vicinity of the fourth to the sixth photosensor elements of a photosensor array. It can be understood that a dark output at a portion where the constant-current source is disposed is larger than other bit outputs and results in fixed-pattern noise.
It is an object of the present invention to provide a photoelectric transducer formed on the same semiconductor substrate which can reduce noise.
It is another object of the present invention to provide a photoelectric transducer having a CMOS constant-current circuit which can reduce fixed-pattern noise when the CMOS constant-current circuit and photosensors are formed on the same semiconductor substrate.
According to one aspect, the present invention which achieves these objectives relates to a photoelectric transducer including a current mirror circuit which includes a first pMOS transistor whose source is connected to a positive power supply, a second pMOS transistor whose source is connected to the positive power supply, and whose gate and drain are connected to the gate of the first pmOS transistor, a first nMOS transistor whose source is connected to a reference potential, and whose gate and drain are connected to the drain of the first pMOS transistor, and a second nMOS transistor whose source is connected to the reference potential via a resistor, and whose gate is connected to the gate of the first nMOS transistor, and whose drain is connected to the drain of the second pMOS transistor, and a plurality of photosensors including a first-conduction-type semiconductor substrate and a plurality of second-conduction-type semiconductor regions formed near a surface of the first-conduction-type semiconductor substrate. The current mirror circuit and the plurality of photosensors are formed on the same semiconductor substrate. Voltage drop means is provided between the drain of the second nMOS transistor and the drain of the second pMOS transistor.
According to another aspect, the present invention which achieves these objectives relates to a photoelectric transducer including a first current mirror circuit which includes pMOS transistors connected to a positive power supply, a second current mirror circuit which includes nMOS transistors connected to a reference potential so as to receive respective currents of the first current mirror circuit, a constant-voltage output circuit for outputting a constant voltage from a connection point of the first current mirror circuit and the second current mirror circuit, a plurality of photosensors including a first-conduction-type semiconductor substrate and a plurality of second-conduction-type second semiconductor regions formed near a surface of the first-conduction-type semiconductor substrate, a source follower circuit for converting photoelectric charges of the photosensors into a voltage and outputting the obtained voltage, and a switching MOS transistor provided between the output of the source follower circuit and the positive power supply. These circuits and components are formed on the same semiconductor substrate. The gate of the switching MOS transistor is connected to the output of the constant-voltage output circuit, and voltage drop means is provided at one of connection points of the first current mirror circuit and the second current mirror circuit.
According to still another aspect, the present invention which achieves these objectives relates to photoelectric transducer including a constant-current-source circuit in which a current mirror circuit including second-conduction-type MOS transistors and a current mirror circuit including first-conduction-type MOS transistors are subjected to tandem connection between power supplies, and a plurality of photosensors formed by a second-conduction-type semiconductor formed near a surface of a first-conduction-type semiconductor substrate. The constant-current-source circuit and the plurality of photosensors are formed on the same semiconductor substrate. Voltage drop means is provided at one of tandem connection portions of the current mirror circuits in order to suppress stray carriers mixed in the plurality of photosensors.
According to yet another aspect, the present invention which achieves these objectives relates to a photoelectric transducer including at least a plurality of transistors including a photosensor unit formed in the same semiconductor chip, and control means for suppressing noise for the photosensor unit due to avalanche multiplication in one of the plurality of transistors.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.